1742 VERNONH. DIBELER AXD
SUS.4N
K. LISTON
It should be noted that the only phosphorus-containing products formed in the reaction of phosphine with chloramine were polymeric phosphorus hydrides in contrast to the apparent red phosphorus formed in the reaction of phosphine with dimethylchloramine. This may be a result of the expected higher basicity of a dimethylamino group compared to that of an amino group. Chloramine cleaves P-P bonds12 and yields a mixture of aminophosphonium chlorides, their condensation products, and P-C1 compounds which are difficult to separate. It is probable that dialkylchloramines (12) S. E. Frazier a n d H H. Sisler, I n o r g C h e m , 6 , 925 (1966).
Inorgnnic Chemistry will also cleave P-P bonds. This may have been the difficulty encountered by Petrov6 in characterizing the products obtained in his experiments. The products reported herein are obtained only when chloramine or dimethylchloramine is not in excess. This type of reaction is currently being investigated with respect to hydrides of other elements and is being extended t o include other substituted phosphines. Acknowledgment.-The support of this research by the National Institutes of Health through Grant S o . CA08263-03 with the University of Florida is gratefully acknowledged.
COKTRIBUTIOS FROM THE INSTITUTE FOR MATERIALS RESEARCH, NATIONAL BUREAUOF STANDARDS, U'ASHINGTOS, D. C. 20234
Mass Spectrometric Study of Photoionization. XII. Boron Trifluoride and Diboron Tetrafluoride BY VERNON H. DIBELER
AND
SUSAS K. LISTOS
Receitied March 29, 1968 Mass spectra and photoion yield curves for the molecular and principal fragment ions of BFa and BzFa are reported. T h e threshold regions only are measured for BzFs+ and B F + from B2F4. These observations are intercompared in order t o derive heats of formation of ions and radicals and bond dissociation energies. The boron-boron bond dissociation energy in B2F4 is calculated to be 431 kJ mol-' (103 kcal mol-1).
Introduction Recent electron impact studies'-4 on BF3 have given remarkably concordant results of 15.5-15.7 eV for the ionization threshold of the molecule in spite of the relatively low intensity of that ion. Unfortunately, no extrapolated Rydberg series or photoionization measurements on BF3 have been reported for comparison. The same a ~ t h o r s l - also ~ report consistent values of about 16.2 eV for the threshold of the BFzf ion in BF3. The ionization energy of BF2 necessary for the calculation of bond energies has not been measured directly. Steele, Nichols, and Stone5 estimated I(BF2) = 9.4 0.1 eV by subtracting an average6 B-F bond energy of BF3 from the electron impact value',* of A(BF2+). On the other hand, Margrave7 used this value and electron impact results to calculate AHf(BF2),thus completing the circular argument. Domalski and ,4rmstrongs have stated that the ther-
*
(1) R . W.Law and J. L. Margrave, J. Chem. P h y s . , 25, 1086 (1956). (2) J. M a r r i o t t a n d J. D. Craggs, J . Eleclvon. Co?ilml, 3, 194 (1957). (3) H. Kreuzer, Z . flaliwfoovsch., 12a, 519 (1957). (4) H. J. Svec in "Mass Spectrometry," R. I. Reed, E d . , Academic Press Inc., New York, N.Y., 1965. ( 5 ) W. C. Steele, L . D. Nichols, and F. G . A. Stone, J .A m . Chem. Soc., 84, 1154 (1962). (6) T. L. Cottrell, "The Strengths of Chemical Bonds," 2nd e d , Butterworth a n d Co. L t d . , London, 1958. (7) J. L. Margrave, J . Phys. Chem., 66, 1209 (1962). ( 8 ) E. S. Domalski a n d G . T.Armstrong, J. Res. N a t i . Bur. S t d . , A71, 195 (1967).
mochemistry of many boron compounds is dependent on the heat of formation of BF3 and of the lower fluorides of boron. Thus the present study was undertaken in an attempt to obtain additional and more accurate values. T o do so, the study includes measurements on B2F4 because of its obvious relationship to BF3 and because it is one of the few volatile boron compounds containing a boron-boron bond. The necessity and advantage of a joint consideration of some of the experimental results for the two molecules will be apparent. The mass spectrum of an apparently impure sample of BsF4 has been reported,g but no published data are available on ionization energies of this molecule. However, Gunn and Greenlo have investigated the heat of reaction for the chlorination of B2F4. The assumed equivalent reaction of BF3 BC13 results in a complex equilibrium mixture, and a B-B bond energy of 72.4 kcal mol-l is calculated from the summation of heats of formation of the components. Unexpectedly, the same method resulted in a B-B bond energy of 79 kcal mol-' for the less stable B2Cl4. The following conversion factors are used in this report: 1 eV molecule-l = 96.4870 kJ mol-]; 1 kcal mol-' = 4.1840 kJ mol-'.
+
(9) J. N. Gayles a n d 1. Self, J. C h e w P h y s . , 40, 3530 (1964). (IO) S. K. G u n n a n d L. C,. Green, J. P h y s . Chmn., 66, 178 (1961).
MASSSPECTROMETRIC STUDY OF PHOTOIONIZATION 1743
Vol. 7, No. 9, September 1,968 Experimental Section The photoionization yield curves and the threshold data were obtained by means of the combined vacuum ultraviolet monochromator and mass spectrometer described previously.11 The continuum photon source, the photon intensities, ion count, and probable uncertainties are essentially as indicated in recent reports.I2 The boron trifluoride was a commercial sample having a minimum purity of 99.5y0 as stated by the supplier. The mass spectrum a t 584 A (21.23 eV) confirmed this limit, showing no measurable impurities of significance to this work. It was used without further purification. The diboron tetrafluoride was prepared by Dr. T. D. Coyle of the Inorganic Chemistry Section by fluorination of B2Ch The crude product was purified by trapwith SbFa a t -80'. to-trap vacuum distillation. Material passing a t -96' but retained at - 125" was transferred to a borosilicate glass storage flask with a freeze-out tip. The material was maintained a t liquid-nitrogen temperature throughout the course of study except for the very brief warmups required for sampling. An infrared spectrum of the purified material showed it to be free of boron chlorides. No BFa was detected and an upper limit of considerably less than 0.5y0is assigned to this impurity. Samples were introduced into the reservoir of the clean stainless steel inlet system of the mass spectrometer by sublimation from the solid. The vapor pressure in the flask and reservoir was never permitted to exceed 5 torr; thus the temperature remained well below -80'. The mass spectrum is not a sensitive means of detecting small quantities of BFa in BgF4. Nevertheless, frequent checks of the mass spectrum of BzFd were made to detect any changes. No significant variation in relative intensities of the appropriate ions was ever observed either during the course of an individual series of measurements or throughout the series of samples removed from the storage flask over a period of 10 days. Thus we assume that no change occurred in the initial composition as given by the infrared measurements.
consistent with previously published electron impact spectra.2 The photoionization efficiency curves for the BF3+ and BFz+ ions are shown in Figure 1. The B F + ion current is too weak to permit a threshold measurement using our continuum photon source. Wavelength in Angstroms is plotted on the abscissa with equivalent energy in electron volts given a t intervals. Ion counts per photon transmitted through the ion source are plotted in arbitrary units on the ordinate. The ordinate scale for the BFz+ ion is five times that of the BF3+ ion. ENERGY (eV) 15 50
Ion
Re1 abundancea
B F3
BF3 + BF2+ BF +
0.085 1.00 70.005
B2F4
B2F4' B2F3 BFI+ BF2 + BF + (21.23 eV).
0.10 1.00 0.008 0.76 0.30
+
a
Measured a t 584 A
Incomplete evidence was obtained for the B F + ion as only one of the isotopic species could be detected. No negative ions were observed and it is estimated that the abundance of any ion-pair process is less than 0.002 relative to the BF2+ ion. These observations are (11) V. H. Diheler and R . M. Reese, J . Res. Natl. Bur. Std., A68, 409 (1 964). (12) V. H.Bibeler, J. A. Walker, and S. K. Liston, ibid., A71, 371 (1967).
1907
I
20 66
I
L
i
t
I
I
BF,
hv -BF:
t
Fte
_.
- 1 I /i'
-
I
1
I
0
Figure 1.-Ionization
Molecule
1771
L I
Results and Discussion BF3.-The relative intensities of the positive ions in the mass spectrum of BF3 obtained by means of 21.23eV photons are given in Table I. The spectrum is obtained a t a constant ion-accelerating voltage of 2.5 kV and the abundances of ions due to the O ' B and IIB isotopes are added to give the monoisotopic spectrum. TABLE I MONOISOTOPIC MASSSPECTRA OF BFI AND BzF4 BY PHOTON ABSORPTION
16 53
efficiency curves for the BF3+ and B F 2 + ions of BFs.
The BF3+ ion yield curve exhibits a slowly rising onset a t 797 (15.55 eV). Although in agreement with recent electron impact results, it probably represents an upper limit to the ionization threshold. Assuming it represents AH0 = 358.6 kcal mol-' for the reaction BF3 hv + BF3+ e and adding the heat of formation13 AHf'o(BF3) = -271.08 kcal mol-', we obtain AHfoo(HF8+) = 87.5 kcal mol-I. Thresholds and derived thermodynamic values are summarized in Table 11. The shape of the BF3+ ion curve near onset suggests the presence of numerous unresolved vibrational levels of the ion, probably resulting from severe distortion of the symmetric planar molecule upon ionization. Thus, i t is reasonable to speculate that no identifiable Rydberg
a
+
+
(13) D.D . Wagman, W. H. Evans, V. B. Parker, L. Halow, S. M. Bailey, and R . H. Schumm, National Bureau of Standards Technical Note 270-3, U. S. Government Printing Office, Washington, D.C.,1968.
1744 VERNONH.DXBELER ANI)
SUMMARY O F
+ hv BIFl + hv BFI
SUSAN
Inorgcrnic Chemistry
K. LISTON
TABLE I1 THRESHOLD ENERGIES AND THERMODYNAMIC PROPERTIES O F IONS AND RADICALS FROM BFs ASD BiFc
Ion process
Threshold, eVa
Thermodynamic property
BF3+ e BF2+ F -t B2F4+4-e -+ B2F3+ F
15.55 i: 0 . 0 4 15.81Sz 0.04 1 2 . 0 7 k 0.01 15.40 & 0 . 0 1
ANr'o(BFB+) = 8 7 . 5 kcal mol-' AHl"o(BF,+) = 7 5 . 1 kcalmol-1 AHtoo(B2F4+)= -65.2 kcal mol-' AHtoo(B2F3+)= - 6 . 8 kcal mol-'; I(BsF8) c 8 . 1 eV AHf'o(BF3) = -120.2 kcal mol-I; Do(BF2-BF2) = 103.1 kcal mol-'; I(BF2) = 8 . 4 7 eV AHioo(BF) = - 2 1 . 9 kcal mol-' AHf"o(BF+) = 221.6 kcal mol-'; I ( B F ) = 10.56 eV
+
BFit
+ + +e + +e + BFl f e
-+
BFl+ BF-
+ BF + F + e + BFa + e
-f
-f
-t
12.94 f 0 . 0 1
18.0 & 0.05 12.75 5 0.01
Estimated uncertainty.
series converging to the first ionization limit will be found in the absorption spectrum of the molecule. The peaks in the wavelength region 765-730 A are ascribed to autoionizing Rydberg levels of the molecule converging to electronically excited states of the ion. A second band of unresolved Rydberg levels appears in the 725-700-& region. The increase in ion yield a t about 690 b (18.0 eV) followed by the relatively smooth continuum to 600 A suggests an ionization threshold of an electronically excited state. ,2 weak but definite onset of the BF2+ion appears at 784 A (15.S1 eV). This is considerably less than the electron impact values a t about 16.2 eV, which is approximately identified with the first maximum of our curve a t 760 b. Taking our threshold of 15.81 eV (364.6 kcal mol-!) to be the A H o for the reaction BF3 hv + BFz+ F e and summing the necessary heats of f ~ r m a t i o n 'we ~ obtain AHfoo(BFz+) = 75.1 kcal mol-'. As noted earlier, no direct measurement of the ionization energy of BF2 is known. However, following Steele, et al., we can immediately approximate i t b y calculating the average B-F bond dissociation energy of BF3 from recent thermodynamic values!3 and subtracting this (6.7 eV) from the threshold energy. Assuming the first B-F bond to be stronger than the average bond, we obtain the upper limit, I(BF2) < 9.1 eV. It is interesting to note that Price and c o ~ o r k e r s ! ~ have compared ionization and dissociation energies of various isoelectronic species and estimated D(BF2-F) = 7.0 eV. Applied to our threshold value of BF2+, this gives an estimated value of I(BF2) = 8.8 eV. Further conclusions concerning this quantity are deferred until the discussion section on BF2+from BzF4. B2F4.-Relative intensities of the positive ions of BzF4 are recorded in Table I. There is no evidence for B2F2f and B2F+ ions or any negative ions a t present sensitivity, and an estimated upper limit to relative intensities is 0.005. The mass spectrum shows no trace of the various impurity ions tabulated by Gayles and Self . g M3th those exceptions, the relative intensities of the principal ions are similar. The weak BF3+ ion is interpreted as a rearrangement ion such as is frequently observed in mass spectra. As such, it limits the usefulness of the
+
+ +
(14) W. C. Price, T. R. Passmore, and D. & Roessler, I. Djscussions Frrra(1963).
day Soc., 96, 201
spectrum as a means of detecting incipient decomposition of BzF4. The ion yield curves for the BzF4+ and BF2+ ions from threshold to 600 b and the initial portions of the BzFs4, BF2+, and BF+ ions are shown in Figures 2 and 3, respectively. The thresholds are summarized in Table I1 Curves were not taken with customary detail because of the scarcity of sample. However, sufficient observations were made on B2F4- and BF2+ to obtain evidence of interesting features.
Figure 2.-Ionization
efficiency curves for the R2F4+ and B F 2 + ions of BzF4.
The B2F4f ion exhibits a definite onset a t 1027 A (12.07 eV). No previous measurement of this ionization threshold is known. It is considerably lower than the ionization energy of BF3or of F2. It is quite similar however to I(B,) = 12.1 eV derived from electron impact studies of diborane.15 Adding the heat of formation13 AHfoo(B2F4)= -343.5 kcal mol-' to our threshold value gives AHfoo(BaFi+) = -65.2 kcal mol-I. The remainder of the ion yield curve shows evidence for unresolved autoionization of Rydberg levels in the molecule. However, there is no important increase in the ionization continuum that could be ascribed to the onset of excited ionic states. (15) W. S. Koski, J. J. Kaufman, C. P. Pachuki, and F. J. Shipko, J . A m . Chem. Soc., 80, 3202 (1958).
Vol. 7 , No. 9, September 1968
MASSSPECTROMETRIC STUDYOF PHOTOIONIZATION 1745 TABLE 111 INDIVIDUAL BONDDISSOCIATION ENERGIES IN BFs AS
DETERMINED FROM BFa
AND
BzF4
Energy--------.
Bond
eV
kcal mol-'
BF2-F BF-Fa B-F5
7.34 5.10 7.53
169.3 117.6 173 6
c
_
Total 19.97 460.5 BF8 -*. B 3 F 19.92b 459 5b Determined for B z F ~ and assumed the same in BFa. b Calculated from heats of formation.13
+
Figure 3.-Initial portions of the ionization efficiency curves for the BzF3+, BFz+, and B F f ions of BzF4.
The ion yield curve of the BF2+ ion is also shown in Figure 2 with the initial onset region replotted on a more sensitive scale in Figure 3. The latter figure shows a definite onset a t 958 fL (12.94 eV). Some evidence for autoionization appears in the wavelength region 840700 A (Figure 2 ) . A t about 690 A (18.0 eV) the ion yield curve rises sharply indicating the onset of a second ionization process. The energy interval of 5.1 eV (118 kcal mol-') is in sufficient agreement with the dissociation energy D(FB-F) = 123 kcal mol-' derived from Knudsen-cell studies by Hildenbrand and MuradL6to identify the dissociative ionization processes to be those given in Table 11. Therefore, we conclude that the lower energy onset of 12.94 eV (298.4 kcal mol-') represents the A H , for the reaction B2F4 hv -+ BF2+ BF2 e. Assuming no excess energy in this reaction and accepting AHfoo(BF2+) = 75.1 kcal mol-' from the BF3 measurements, we calculate AHfoo(BF2) = -120.2 kcal mol-', from which we immediately obtain Do(F2B-BFz) = 103.1 kcal mol-'. This is greater than the value of 72.4 kcal mol-' obtained by Gunn and Greenlo b u t is consistent with the relative stability of B2F4 compared with BzC14. We also note that our B-B bond value is similar to the B-C bond strength of -100 kcal mol-' obtained by Steele, et ~ l . from , ~ alkyl-substituted boron difluorides. The difference between AHfoo(BF2+) and A H f o o (BF2) is, of course, the ionization energy of BF2. The above values for these properties permit us to talculate I(BF2) = 195.3 kcal mol-', or 8.47 eV, and, for the BF3 molecule, D(BF2-F) = 169.3 kcal mol-', or 7.34 eV. These are generally in agreement with, but much more accurate than, the previous estimates of these properties outlined earlier. Bond dissociation energies of BF3 are summarized in Table 111. The second onset of the BF2+ ion a t 18.0 eV (Figure 2) is ascribed to the process BzF4 hv BF2+ B F F e. If we assume 18.0 eV (415.1 kcal mol-') represents AH0 for the reaction, we can then calculate A H f o o (BF) = -21.9 kcal mol-' and, obviously, Do(BF) = 173.6 kcal mol-', or 7.53 eV. This is somewhat lower
+
+
+
+
+
+
+
(16) D. L. Hildenbrand and E. M u r a d , J . Chem. Phys., 43, 1400 (1965).
than Do= 7.85 f 0.15 eV reported by Hildenbrand and Murad and the spectroscopic value of 185 kcal mol-' favored by Barrow." Although the uncertainty in our value is difficult to assess, in the absence of systematic error we estimate about 1 kcal mol-' or 0.04 eV. This is indicated by a comparison of the sum of the individual B-F bond dissociation energies obtained from BF3 and BzF4 with the heat of atomization of BF3 calculated from the appropriate heats of formation. As shown in Table 111, the agreement is quite satisfactory, although, of course, correct distribution among the bonds is not proved. The B F + ion of B z F (Figure ~ 3) shows a very definite onset at 972 A (12.75 eV). The low energy indicates a very stable neutral product. If the threshold energy (294.0 kcal mol-') is taken to be the AH0 for the reaction BzF4 hv .--+ B F + BF3 e, we calculate AH*', (BF+) = 221.6 kcal mol-l. Subtracting the above heat of formation of B F gives I(BF) = 243.5 kcal mol-', or 10.56 eV. This differs considerably from the ionization energies of the isoelectronic molecules Nz (15.75 eV) and CO (14.01 eV). However, the assumed process is very probably correct as any other would result in a still lower I(BF). The value of AHf(BF+) precludes the observations of B F + from BF3 at 584 A, assuming molecular fluorine as the neutral product. However, we have some evidence for the ion in BF3 and we cannot rule out the possibility that AHf(BF+) may be an upper boundary to the true value. The B2F3+ ion of BzF4 shows a very definite onset at 805 A (15.40 eV). Assuming that 355.1 kcal mol-' is the AH0 for the reaction B2F4 hv BzF3+ F, we calculate AHroo(B2F3+) = -6.8 kcal mol-'. Assuming that D(BaF3-F) is approximately the same as D(BF2-F), we estimate I(BZF3) = 8.1 eV. This is very near our value of I(BF2). Unfortunately, these last results cannot be compared with independent measurements. While we assume the near equivalence of the first B-F bond dissociation energy in the neutral molecules, it should be noted that the dissociation energy for the same bond calculated for the ions BF3+ and B2F4+ differs widely (6.0 and 76.8 kcal mol-', respectively). Fundamentally, this is related t o the very different ionization energies of the two molecules and suggests
+
+
+
+
-+
(17) R. F. Barrow, Trans. F a r a d a y Soc., 56, 952 (1960)
+
1746 EVANS,HOLLOWAY, SUKUMARABANDHU, AND MCDANIEL that ionization of BzF4 occurs through removal of an electron much more closely associated with the B-B skeleton than with the B-F bond. Acknowledgment.-The
authors gratefully acknowl-
Inorganic Chemistry
edge valuable discussions on thermodynamic properties with Dr. W. H. Evans and the contribution made by Dr. T. D. Coyle through the generous gift of the BzF4. This work was supported in part by the U. 8 . Atomic Energy Commission.
COXTRIBUTIOX FROM THE DEPARIMENT O F CHEMISTRY, UNIVERSITY OF CISCINSATI,CINCINSATI, OHIO 45221
Strong Hydrogen Bonds.
IV.
The B2H,-
Species1'2
BY WILBUR G. EVASS, CLIVE E . HOLLOWAY, KAMTHORX SUKUMARABANDHU, DARL H. MCDANIEL
AND
Received September 12, 1967 Tetraethylammonium borohydride reacts a t - 78.5" with diborane to form ( C S H ~ ) ~ N B ~At H ~0". hydrogen is also produced in a 1:10 mol ratio of H2 to ( C ~ H S ) ~ B HCalorimetric ~, studies allow an estimate of -31 =t= 8 kcal/mol to be made for the minimum enthalpy of formation of the hydrogen bond in B ~ H T - . This large enthalpy for an electron-deficient hydrogen bond is interpreted as showing that the lone pair usually deemed necessary for hydrogen-bond formation may actually be antibonding in character.
The B2H7- species was first postulated by Brown, Tierney, and Stehle3 t o account for the deviation from Henry's law of the pressure of B2H6in contact with a solution of NaBHJ in d i g l ~ m e . They ~ proposed a structure for this anion involving a single hydrogen bridge bond. This is the only structure consistent with the equations of balance5 for this species. Our interest in the BzH7- species centered about the hydrogen-bond energy of the electron-deficient BHB bond. I n order to obtain a value for this energy, a first requirement is a simple borohydride salt which reacts with diborane with no solvent present. A solid NaBzH7.diglyme had previously been isolated but attempts to remove the diglyme had been unsuccessfuL6 Lipscomb' had suggested that the BzH7- ion may not exist in the absence of solvent stabilization. If the role of the diglyme is to solvate the sodium ion, rather than the B2H7- ion, removal of the diglyme should produce a salt which could increase its lattice energy greatly through the loss of diborane iTaB2H7(s)+NaBHa(s) -I- O,5B,HB(g)
According to this viewpoint, the reverse of the above reaction does not occur (in the absence of solvent), because the energy needed to expand the NaBH? lattice in order to accommodate the B2H7- species which would be formed is greater than the energy which is released through the formation of the B2H7- species. The (1) T a k e n i n p a r t from t h e P h . D . thesis of W.G. E v a n s , 1964. (2) Paper 111: D. H. McDaniel and U'. G. E v a n s , I?tovg. C h e m . , 6, 2180 (1966). (3) H . C . Brown, P. F.Stehle, and P . A. Tierney, J . A m . Chem. Soc., 79, 2020 (1957). (4) Dimethyl ether of diethylene glycol. ( 5 ) W. N. Lipscomb, "Boron Hydrides," W. A. Benjamin, Inc., S e w York, 3 . Y., 1964, p 43. (6) E . B. Baker, R. B. Ellis, a n d W.S . Wilcox, J . Inorg. . V z d C h e m . , 28, 41 (1961). (7) See ref 5, p 130.
situation appears to be analogous to the failure of NaBr to add HBr or XaI to add HI although such reactions do occur when tetraalkylammonium salts are used instead of sodium salts. The ionic radiusR of BH4- is 2.03 A,compared to 1.93 A for Br- and 2.16 A for I-. Adamsg was unable to prepare KBaH7 either in solution or as a solid, probably because the difference in lattice energies of KBHr and KBzH7 is large and the energy of coordination of potassium ion with diglyme is small. It would appear that stable compounds containing the BPH7- species could be synthesized if the size of the cation were adjusted so that the lattice energies of MBH4 and hIB2H7were comparable. The tetraalkylammonium cation seemed to be most suitable for this purpose. It was expected that very little lattice expansion would be needed to insert a "BH3" unit into the lattice of a tetraalkylammonium borohydride. The preparation of a B e H y compound under these circumstances would indicate that stabilization by solvation is not essential. Experimental Section Tetraethylammonium borohydridc (hldrich Chemical Co.), reported purity 97.570, was used as received. Diborane was prepared from the reaction of concentrated H2SOa with KBHt (Callery Chemical Co.) under conditions described by Jolly.'O Pressure-composition isotherms were obtained a t - i 8 . 5 and 0' in a greaseless section of the vacuum line." At -78.5" (see Figure 1) the reaction between B.Ha and (C2Hj)JYBHq evolved a (8) S . C. Abrahams and J. Kalnajs, J . C h e m . P h y s . , 22, 434 (1954); t h e B r - and I- radii are from L, Pauling, " T h e X a t u r e of t h e Chemical Bond," 3rd ed, Cornel1 University Press, I t h a c a , h-. Y . , 1960, p 514. (9) R . M. Adams, "Borax t o Boranes," Advances in Chemistry Series, No. 32, American Chemical Society, Washington, D. C., 1901, p 67. (10) W. J. Jolly, "Synthetic Inorganic Chemistry." Prentice-Hall, Englewood Cliffs, h-.J., 1960, p 158. (11) F o r a general discussion of vacuum-line techniques see R . T. Sanderson, "Vacuum Manipulation of Volatile Compounds," John Wiley and Sons, Inc., Xew York, N. Y., 1948.
